Ion source

ABSTRACT

An electrically conductive, high-transmission membrane is supported between an ionization region and an exit region for positive ions in an ion source. The membrane is operated at a potential more negative than the source of ionizing electrons to repel primary and secondary electrons exiting from the ion source while transmitting the desired positive ions.

UnitedlStates Patent 1191 Kruger et al.

[ ION SOURCE [75] Inventors: William P. Kruger, Los Altos Hills;

Wilson R. Turner, Los Gatos; Robert D. Board, Sunnyvale, all of Calif.

[73] Assignee: Hewlett-Packard Company, Palo Alto, Calif.

[22] Filed: Nov. 9, 1972 [21] Appl. No.: 306,223

[52] US. Cl. 250/427, 313/63 [51] Int. Cl. H01j 39/34 [58] Field of Search 250/423, 427;

[56] References Cited UNITED STATES PATENTS 3,723,729 3/1973 Kruger et al 313/63 X Oct. 16,1973

3,221,164 11/1965 Gunther .Q. 250/427 2,973,444 2/1961 Dewan ..s1a/eax Primary Examiner-James W. Lawrence AttorneyPatrick J. Barrett [57] ABSTRACT than the source of ionizing electrons to repel primary and secondary electrons exiting from the ion source while transmitting the desired positive ions.

3 Claims, 2 Drawing Figures ION SOURCE BACKGROUND AND SUMMARY OF THE INVENTION Mass spectrometers, particularly those used to analyze organic molecules by electron-impact fragmentation and ionization, usually suffer from various contamination problems. One particularly difficult contamination problem is the formation of thin, tough, insulative layers on the various metallic component in the analyzing portion of the spectrometer. These insulative layers are often formed by polymerization of the various organic molecules and fragments present in the system by high energy electrons emerging from the ionization chamber. Stray charge patterns accumulate on these insulative layers and disrupt the operation of the analyzing portion of the spectrometer. The only way to remove the unpredictable and disturbing effects of these stray charge patterns is to disassemble the spectrometer and clean the contaminated elements, an expensive and time consuming process.

The present invention significantly reduces the number of high energy electrons that escape from the ionization chamber into other portions of the mass spectrometer. A high transmission membrane is located between the ionization region and the exit region for positive ions in the ionization chamber. This membrane is maintained at a sufficiently negative potential to prevent both primary electrons from the electron source and secondary electrons generated in the ionization region from exiting along with the desired positive ions. The operation of the membrane is independent of the ion focusing elements in the exit region. Support for the membrane may be provided either independent of the other elements in the ionization chamber or may be by attachment to electron focusing electrodes near the filaments supplying the bombarding electrons.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cross-sectional view of an ionization chamber mounted in a mass spectrometer;

FIG. 2 shows a modified cross-section of the ionization chamber in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT An ionization chamber is shown in FIG. I mounted in a portion 9 of a mass spectrometer 11. An example of such an ionization chamber is disclosed in patent application entitled Ionization Chamber, Ser. Number 111,910 filed Feb. 2, 1971, by William P. Kruger and Wilson R. Turner and assigned to the'assignee of the instant application. Although the invention will be described in conjunction with the ionization chamber therein disclosed, it should be understood that the invention can be used with most ionization chambers suitable for use in mass spectrometers. Typically, spectrometer 11 will have various metallic electrodes, such as entrance aperture or slit l3 and element 15, as well as other components omitted from the drawing for the sake of clarity.

Ionization chamber 10 has a pair of filaments 26 and 26', and a pair of electron focusing electrodes 28 and 28. Membranes 16 and 18 supported on electrodes 12 and 14 respectively define an ionization region 20. Samples to be ionized are introduced into the ionization region through tube 22 and are bombarded with electrons from filaments 26. Potentials are applied to electrodes 12 and 14 to propel positive ions out of the ionization region and toward the right in FIG. 1, into elements 13 and 15.

As previously mentioned, organic samples may be polymerized into thin, tough, insulative layers on elements 13 and 15 by stray electrons from the ionization chamber, unless such electrons are restrained from exiting from the ionization chamber. An electrode 17 is supported by a conductive support element 30a in an exit region 21 of the ionization chamber. Across one end of electrode 17 is a permeable membrane 19 through which most of the positive ions from the ionization region pass.

When electrode 17 and membrane 19 are made more negative in potential than filaments 26, electrons exiting along with the positive ions will be repelled back toward ionization region 20. This repulsion will affect both primary electrons coming directly from the filaments and secondary electrons that have been generated by collisions of primary electrons with sample moleculesin the ionization region or with other electrodes. Since high energy electrons are necessary for the formation of the undesirable insulative layers on elements such as 13 and 15, the repulsion of primary and secondary electrons by electrode 17 and membrane 19 will effectively prevent formation of those layers.

FIG. 2 illustrates an alternative way of supporting membrane 19in which electron focusing electrodes 28 and 28' have been extended to support membrane 19. Since the electron focusing electrodes are maintained at a potential more negative than filaments 26 and 26' in ordei to focus electrons into the ionization region, membrane 19 will repel primary and secondary electrons.

We claim:

1. In an ionization chamber for a mass spectrometer including an electron source, an ionization region in which s ample molecules are ionized through collisions with electrons from the electron source, and ion propulsionele'ctrodes for propelling positive ions out of the ionization region through an exit region, the improveme'nt comprising an electron repelling electrode situated between the ionization region and'the exit region, the electron repellingelectrode being maintained at a potential more negative than the electron source.

2. An ionization chamber as in claim 1 wherein the electron repelling electrode comprises a permeable cusing electrodes. 

1. In an ionization chamber for a mass spectrometer including an electron source, an ionization region in which sample molecules are ionized through collisions with electrons from the electron source, and ion propulsion electrodes for propelling positive ions out of the ionization region through an exit region, the improvement comprising an electron repelling electrode situated between the ionization region and the exit region, the electron repelling electrode being maintained at a potential more negative than the electron source.
 2. An ionization chamber as in claim 1 wherein the electron repelling electrode comprises a permeable membrane supported between the ionization region and the exit region.
 3. An ionization chamber as in claim 2 including electron focusing electrodes in the electron source wherein the membrane is supported by the electron focusing electrodes. 